1. Field of the Invention
The invention relates to an apparatus for the indication of the fuel consumption per distance unit of a motor vehicle, and more particularly, to such an apparatus with means to compensate for different gears selected for the vehicle.
2. Prior Art
In such known devices for the indication of the fuel consumption of an Otto engine per distance unit by means of a pressure meter, especially an indication manometer, the basic fact is that a fixed relation exists between the vacuum prevailing in the suction pipe of the Otto engine, the suction pipe pressure, and the fuel consumption per distance unit. Such a fixed interrelation, according to which the scale of the indication manometer is initially calibrated, exists, however, only for a prespecified gear ratio between the crankshaft of the Otto engine and the drive wheels. In other words, the fuel consumption per distance unit, which in this case is usually to be indicated in liters per 100 km, depends in a motor vehicle with a gearshift mechanism on the gear inserted at each instance. This means that the correct fuel consumption per distance unit can be measured exactly in an absolute value for a specific gear only. Ordinarily, this is the direct, that is to say, the fourth gear. For the smaller gears, to be sure, the indication is still correct in its tendency, but cannot give the driver the important information about the correct gear choice, since in the shifting-back from the direct gear into the smaller gear, a decrease in consumption is always indicated, although in fact, a substantial increase in consumption may occur.
In order to overcome this drawback, it is known to switch indicating manometers by electrically controlled means for influencing the indication in dependency on the inserted gear of the gearshift mechanism, to the effective position for the direct gear only but not for the lower gears. In detail, this takes place by a ventilation aperture in the connecting piece of the indication manometer, which aperture can be closed by a closure element actuated by a folding knife-edge relay. The folding knife-edge relay is arranged in the circuit of a battery in which a key switch is also inserted which is connected with the gear choice lever in such a way that the folding knife-edge relay lifts the closure element from the ventilation aperture when the direct gear is not inserted. Thereby, indication errors are avoided. It is in this case, however, disadvantageous that the driver is informed on fuel consumption only when the direct gear is inserted, that is to say, when one gear is inserted. In this known apparatus there may be the further disadvantage that with insertion of the direct gear (notwithstanding the closing of the ventilation bore by the closure element) the indication does not become effective immediately since in the tube spring of the indication manometer the vacuum corresponding to the suction pipe pressure must first be built up.
A correct indication of the fuel consumption per distance unit for two selectively insertable gears is achieved, in another apparatus belonging to prior art, by arranging the suction pipe and the indication manometer a pneumatic pressure converter, controlled in dependency on the position of the shift lever. This pressure converter operates in detail according to the principle of power compensation by means of a valve device actuated by a double diaphragm system. This valve device changes the relation between the suction pipe pressure and the pressure conducted to the indication manometer in the ratio of the diaphragm surface acted upon by these pressures. The ratio of the pressure responsive diaphragm surface thus presents the ratio of the multiplications between the direct and the following lower gears.
This apparatus which belongs to the prior art permits, to be sure, a correct indication of the fuel consumption per distance unit in a scaling of the indication manometer for two gears. It is, however, due to the electromagnetically controllable pneumatic pressure converter, relatively expensive.
An inexpensive apparatus for the indication of fuel consumption per distance unit which applies to several gears can be produced by associating with each gear stage a scale of the indication manometer. This multiple scaling of the indication manometer, however, is relatively complex and requires too much attention by the driver.